Measure paint task buffer maps in the memory profiler.

Example output from the memory profiler:

```
|       1.04 MiB -- url(http://en.wikipedia.org/wiki/Main_Page)
|          0.26 MiB -- display-list
|          0.78 MiB -- paint-task       # new output line
|             0.78 MiB -- buffer-map    # new output line
```

The buffer maps aren't huge, but they're worth measuring, and it's good
to get the memory profiler plumbing into PaintTask.

components/compositing/pipeline.rs

@@ -136,12 +136,15 @@ impl Pipeline {
         };
 
         PaintTask::create(id,
+                          load_data.url.clone(),
+                          paint_chan.clone(),
                           paint_port,
                           compositor_proxy,
                           constellation_chan.clone(),
                           font_cache_task.clone(),
                           failure.clone(),
                           time_profiler_chan.clone(),
+                          mem_profiler_chan.clone(),
                           paint_shutdown_chan);
 
         LayoutTaskFactory::create(None::<&mut LTF>,